The present invention relates to digital storage devices and, more particularly, to disk drives which monitor the flying height of dual element transducers that employ magneto-resistive (MR) read elements.
A disk drive is a digital data storage device that stores information within concentric tracks on a storage disk. The storage disk is coated on both of its primary surfaces with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field. During operation of a disk drive, the disk is rotated about a central axis at a constant rate. To read data from or write data to the disk, a magnetic transducer (or head) is positioned above (or below) a desired track of the disk while the disk is spinning.
Writing is performed by delivering a polarity-switching write current signal to the transducer while the transducer is positioned above (or below) the desired track. The write signal creates a variable magnetic field at a gap portion of the transducer that induces magnetically polarized transitions into the desired track. The magnetically polarized transitions are representative of the data being stored.
Reading is performed by sensing the magnetically polarized transitions on a track with the transducer. As the disk spins below (or above) the transducer, the magnetically polarized transitions on the track induce a varying magnetic field into the transducer. The transducer converts the varying magnetic field into an analog read signal that is delivered to a preamplifier and then to a read channel for appropriate processing. The read channel converts the analog read signal into a digital signal that is processed and then provided by a controller to a host computer system.
FIG. 1 illustrates a standard disk drive, generally designated 10. The disk drive 10 includes a disk 12 that is rotated by a spin motor 14. The spin motor 14 is mounted to a base plate 16. An actuator arm assembly 18 is also mounted to the base plate 16.
The actuator arm assembly 18 includes a transducer 20 mounted to a flexure arm 22, which is attached to an actuator arm 24 that can rotate about a bearing assembly 26. The actuator arm assembly 18 includes a voice coil motor (VCM) 28, which radially positions the transducer 20 relative to the disk 12. The spin motor 14, transducer 20 and VCM 28 are coupled to electronic circuits 30 mounted to a printed circuit board 32. The electronic circuits 30 typically include a preamplifier, a read channel, a servo control unit, a microprocessor-based controller and a random access memory (RAM).
The disk drive 10 may include a plurality of disks 12, each with two recording surfaces. In this case, two actuator arm assemblies 18 are provided for each disk 12.
The transducer 20 is a dual element transducer that includes separate read and write elements. Single element transducers usually contain a single inductive element that performs both read and write functions, whereas dual element transducers usually contain a magneto-resistive (MR) read element and an inductive write element. The MR read element can be a conventional magneto-resistive element, a giant magneto-resistive (GMR) element, or a similar component.
Since the transducer 20 is a dual element transducer, the read and write elements can be optimized for their respective functions. For example, MR read elements are more sensitive than inductive read elements to small variable magnetic fields, which permits MR read elements to read much fainter signals from the disk surface. Employing an MR read element permits data to be more densely packed on the disk surface.
MR read elements generally include a strip of magneto-resistive material between two magnetic shields. When properly biased, the resistance of the magneto-resistive material varies almost linearly with an applied magnetic field. During a read operation, the MR strip is positioned above (or below) a desired track within the varying magnetic field caused by magnetic transitions on the track and a constant bias current is passed through the strip. By Ohm""s law (V=IR), the variable resistance and the constant bias current of the MR strip result in a variable voltage across the MR strip that is proportional to the variable resistance. That is, V+xcex4V=I(R+xcex4R). Therefore, the variable voltage is representative of the data stored within the desired track. The variable voltage provides an analog read signal which is then amplified by the preamplifier, processed and converted into digital form by the read channel, and transferred by the controller to a host computer.
FIG. 2 is a diagrammatic representation of an air bearing surface of the transducer 20 which faces the disk 12. As is seen, the transducer 20 includes an inductive write element 34, a write gap 36, a first shield 38, a second shield 40, a read gap 42, and an MR read element 44.
During a read operation, the magnetically polarized transitions previously written onto the disk 12 are read by the MR read element 44. The first and second shields 38 and 40 form the read gap 42 which serves to focus the flux from the magnetically polarized transitions onto the MR read element 44 by shielding the MR element 44 from other sources of magnetic flux (e.g., sources of magnetic flux not associated with the particular location from which information is being read). In other words, the first and second shields 38 and 40 shunt extraneous magnetic flux away from the MR read element 44 as reading occurs.
During a write operation, variable current is applied to write coils (not shown) in the transducer 20 which induce magnetic flux across the write gap 36 between the write element 34 and the first shield 38. The write element 34 and first shield 38 act as poles for an electromagnet which induces the magnetic flux across the write gap 36 that records magnetically polarized transitions on the disk 12. Furthermore, since the magnetic flux in the write gap 36 has relatively high intensity, and the MR read element 44 is in close proximity to the write gap 36, a large amount of the magnetic flux across the write gap 36 strikes the MR read element 44 during a write operation. Consequently, the MR read element 44 is typically not used to read data from the disk 12 during a write operation.
FIG. 3 is a simplified diagrammatic representation of a cross-sectional view of an air bearing slider 46 that includes the transducer 20 flying above a disk surface 48 of the disk 12. The slider 46 is located at the distal end (opposite VCM 28) of the actuator arm assembly 18. The slider 46 includes a leading edge 50 and a trailing edge 52. The transducer 20 is located proximate the trailing edge 52.
During operation of the disk drive 10, the disk 12 is rotated in the direction of arrow A from the leading edge 50 to the trailing edge 52. The slider 46 is aerodynamically designed so that when the disk 12 revolves at its normal operating speed, a small cushion of air between the slider 46 and the disk surface 48 lifts the slider 46 (and hence the transducer 20) a predetermined distance above the disk surface 48. The distance between the transducer 20 and the disk surface 48 is known as the flying height (hf) of the transducer 20. The performance of the disk drive 10 will depend, to a large extent, on whether the flying height of the transducer 20 stays within a predetermined flying height range. For instance, if the flying height of transducer 20 is too low then transducer 20 might crash, engage in excessive contact with the disk surface 48 resulting in damage to the transducer 20 and/or disk 12, or accumulate excessive debris or lubricant from disk surface 48. On the other hand, if the flying height of transducer 20 is too high then data errors might occur during read and write operations. More particularly, if the transducer 20 flies too high during a read operation then the transducer 20 might not adequately sense the magnetic polarity transitions on the disk 12, and if the transducer 20 flies too high during a write operation then the transducer 20 might not adequately induce the magnetic polarity transitions onto the disk 12.
There are two main causes of unwanted changes in flying height. First, the slider 46 may strike contaminants 54 on the disk surface 48 which temporarily stick to the slider 46 and change its aerodynamic characteristics. Second, the slider 46 may strike and bounce off contaminants 54 or aberrations 56 in the disk surface 48. In addition, the flying height may change for other reasons. For instance, gradual accumulation of debris onto the slider 46 can increase the flying height.
As mentioned above, unexpected changes in flying height can result in performance and/or reliability degradation of the disk drive 10. The present invention is directed to alleviating the problem of high fly writing which occurs when the disk drive 10 performs a write operation while the transducer 20 flies too high.
FIG. 4 is a simplified diagrammatic representation of a cross-sectional view of the slider 46 during high fly writing. As is seen, the flying height (hf) of the transducer 20 exceeds a predetermined maximum flying height (hmax) by a distance x. In other words, hf=hmax+x. Since the write element 34 is higher than the predetermined maximum flying height, the magnetically polarized transitions (data) written onto the disk surface 48 are faintly or poorly written. Consequently, the poorly written data is not properly read by the MR read element 44 when such data is sought to be recovered. In addition, since the write element 34 is higher than the predetermined maximum flying height, the write element 34 may also write over parts of tracks adjacent to the track onto which the data is sought to be written. This may render previously written data on the adjacent tracks to be unreadable.
For a more complete understanding of the present invention, a discussion of thermally induced signals in MR read elements is presented. The resistance of MR read elements varies not only in response to an applied magnetic field, but also in response to temperature changes. That is, the resistance of the MR read element is temperature dependent. Accordingly, an increase in the temperature (T+xcex4T) of the MR read element increases the resistance (R+xcex4R) of the MR read element. Since the analog read signal (in volts) is proportional to the resistance of the MR read element, increasing the temperature of the MR read element causes the analog read voltage to increase.
There are several known phenomena that cause thermally induced signals in MR read elements during disk drive operations. For instance, when the MR read element 44 strikes contaminants 54 or aberrations 56 on the disk surface 48, the temperature of MR read element 44 rises and creates a thermally induced signal known as a thermal asperity. As another example, the bias current that flows through the MR read element 44 increases the temperature of the MR read element 44, however the disk 12 operates at essentially the ambient temperature. As a result, the disk 12 provides a heat sink for the MR read element 44. The ability of the disk 12 to draw heat from the MR read element 44 is related to the distance between the disk 12 and the MR read element 44. Therefore, as the flying height of the MR read element changes, the temperature of the MR read element 44 also changes, which causes a thermally induced signal in the MR read element 44.
Conventional disk drives monitor thermal asperities during read operations in order to prevent read errors. For instance, the detection of a thermal asperity during a read operation may result in an error recovery operation.
Furthermore, in conventional disk drives, the bias current is applied to the MR read element during read and write operations to ensure that the MR read element is maintained at a relatively steady-state temperature to avoid unwanted thermally induced signals during the read operation.
However, in conventional disk drives with dual element transducers, the MR read element is often in close proximity to the write element and not used to read data from the disk during a write operation due to interference from the magnetic flux generated by the write element during the write operation. In other words, the MR read element would be disrupted in its normally useable frequency range by the magnetic fields generated by the write element, which would prevent the MR read element from providing an analog read signal indicative of data being read from the disk.
Accordingly, a need exists for a disk drive with a dual element transducer that monitors flying height during a write operation so that appropriate measures can be taken when high fly writing occurs.
In accordance with the invention, a flying height monitoring system is disclosed. Generally speaking, the present invention monitors the flying height of a dual element transducer by monitoring a thermally induced signal generated by the read element during a write operation. The thermally induced signal can be separated from a magnetically induced signal using a filter. For instance, data storage systems with sufficiently high data transfer rates create magnetically induced signals with higher frequency contents than thermally induced signals. In this instance, a low pass filter coupled to the read element can isolate the thermally induced signal from the magnetically induced signal, and a detector coupled to the filter can determine whether the thermally induced signal exceeds a threshold value. If the thermally induced signal exceeds the threshold value, the detector can send a warning signal to a controller so that appropriate measures, such as repeating the write operation, can be taken.
In one embodiment, a flying height monitoring system includes a recording media for storing information, a transducer including a write element for writing information to the recording media and a read element for reading information from the recording media, and a monitoring circuit for monitoring a thermally induced signal generated by the read element while the write element is writing information to the recording media.
In another embodiment, a disk drive includes a disk for storing information and an air bearing slider with a transducer that includes a write element for writing information to the disk during a write operation and a magneto-resistive (MR) read element for reading information from the disk during a read operation. The MR read element generates a readback signal during the write operation that includes a thermally induced signal caused by thermal changes in the MR read element and a magnetically induced signal caused by magnetic flux applied by the write element to the MR read element. The disk drive also includes a filter for receiving the readback signal and isolating the thermally induced signal from the magnetically induced signal, a threshold detector for receiving the thermally induced signal from the filter and generating a high fly write detection signal when the thermally induced signal exceeds a threshold value, and a controller for modifying the write operation in response to the high fly write detection signal.
Preferably, the thermally induced signal has a frequency content of at most 3 MHz, the magnetically induced signal has a frequency content of at least 5 MHz, and the filter is a low pass filter. The thermally induced signal can be a thermal asperity which occurs when the transducer strikes an object, such as contamination or an aberration on the disk. Alternatively, the thermally induced signal can occur due to a change in flying height when the disk provides a heat sink for the transducer.
In accordance with another aspect of the invention, the controller stores a first set of data related to the thermally induced signal provided by the MR read element at a first time during normal operation of the disk drive, the controller stores a second set of data related to the thermally induced signal provided by the MR read element at a second time during normal operation of the disk drive, and the controller compares the first and second sets of data to determine whether debris has accumulated on the transducer. In the event the controller determines that debris has accumulated, the controller can issue a command that causes the transducer to vibrate in an effort to shake off the accumulated debris, or alternatively, the controller can notify a user.
These and other objects, features and advantages of the invention will be apparent from the following specification taken in conjunction with the following drawings.